1. Field of the Invention
This invention relates to novel bulk materials doped in a manner to cause rapid change from an electrically conductive portion to an electrically insulative portion resulting from a small change in the doping profile.
2. Brief Description of the Prior Art
No prior art is known wherein a bulk material changes rapidly from electrically insulating to electrically conductive or vice versa as a result of a small change in doping profile thereof.
It is known from the above noted copending application that Si, Ge, Group III-V and Group II-VI compounds and particularly gallium arsenide (GaAs) and gallium phosphide (GaP), if appropriately fabricated (i.e, grown and doped) provide excellent electrooptical (EO) window and dome candidate materials for infrared (1 to 14 .mu.m wavelength) transparency. Both GaAs and GaP can be doped with a shallow donor in an amount from about 5.times.10.sup.15 atoms/cc to about 2.times.10.sup.16 atoms/cc with a preferred amount of 8.times.10.sup.15 atoms/cc to render the materials conductive with resistivities up to about 0.1 ohm-cm. The above noted copending application discusses GaAs with desired resistivity of from about 0.07 to about 10 ohm-cm with a preferred resistivity of about 0.1 ohm-cm and an electron mobility of greater than about 3000 cm.sup.2 /volt-second and preferably about 5000 cm.sup.2 /volt-second. If the amount of carbon in the melt is greater than 1.times.10.sup.7 atoms/cc, then an increased amount of Se must be used, such as about 5.times.10.sup.16 atoms/cc with inferior results. The preferred shallow donor for GaAs or GaP is selenium (Se), though tellurium (Te) and sulfur (S) can also be used with inferior results (less uniformity) since Se, which segregates less during growth, fits into the lattice structure of GaAs and GaP better than do Te or S. The shallow donor used, of course, must be matched to the lattice structure of the material with which it will be associated.
A shallow donor is one wherein the amount of energy required to ionize or remove an electron from the outer or valence band is extremely low, the energy at room temperature being more than sufficient to pull the electron off, creating a conduction electron. The conduction electrons have high mobility and result in free-carrier absorption in the material. The high mobility of the conductive electrons creates a large dependence of the free-carrier absorption on wavelength whereby, at longer wavelengths (e.g. about 10 GHz range), the material doped with the shallow donor becomes very highly absorbing with effectively no absorption at lower wavelengths (i.e., the infrared or 1 to 14 micron range).
Another reason for selecting shallow donors is that they are fully ionized at room temperature. At room temperature, the shallow donors will have contributed all of their donor electrons to the conduction band. Therefore, if the temperature of the material changes, there is little change in the physical properties of the material. This conductivity can be controlled so that the GaAs and GaP remain infrared transparent in the IR frequency range while offering substantial electro-magnetic interference (EMI) and electro-magnetic pulse (EMP)(EMI/EMP) protection or shielding and being opaque to frequencies outside the infrared frequency range of interest (i.e., 1 to 14 microns). This protection is due to the coupling of the EMI/EMP to the free carriers in the GaAs or GaP. This coupling causes reflection and, to a much greater extent, absorption of the EMI/EMP. Specifically, n-type GaAs with a resistivity of about 0.1 ohm-cm and high electron mobility greater than 5000 cm.sup.2 /volt-second has been fabricated by doping with selenium, though tellurium (Te) and sulfur (S) can also be used, resulting in a material with measured optical and EMI/EMP properties as follows:
______________________________________ n-Type GaAs EMI/EMP Window/Dome Properties Optical Absorption Coefficients (cm.sup.-1) Attenuation @ 15 GHz (dB) ______________________________________ &lt;0.02 &gt;60 ______________________________________
This highly transparent and EMI/EMP shielding GaAs has no modulation transfer function (MTF) losses. Similar properties are achieved in GaP. These materials provide the solution to the problem of infrared windows and domes with inadequate EMI/EMP protection for EO systems.
The crystal growth as described in the above noted copending application requires that such growth start at the bottom and preferably at the center of the melt and gradually continue outwardly and upwardly toward the sides of a tapered vessel and then only upwardly when the crystal growth has reached the sides of the vessel. Such crystal growth causes a large portion of the impurities in the melt, including a large portion of the shallow donor, to move upwardly in the melt. The result is that the shallow donor doping concentration increases in an upward direction or direction of crystal formation because the donor concentration in the melt continually increases as more of the melt is crystallized.